33 



3.2.4 Apparent RPD Depth 



The frequency distribution of apparent RPD depths for the 25 WLIS "F" REMOTS® 

 stations had a major mode in the 2.0-2.5 cm range and a mean of 2.3 cm (Figure 3-15). The 

 frequency distribution of the 1991 RPD depth data had a similar major mode in the 2.0- 

 2.5 cm range and a mean of 2.16 cm. Analysis of the 1992 RPD data revealed a relatively 

 even distribution of the number of stations within each 0.5 cm depth interval range for 

 apparent RPD depths of 1.0 to 3.5 cm. This broad distribution of the 1992 RPD depth data 

 was in contrast to the 1991 survey, in which mean apparent RPD depth values were clustered 

 in the 2.0-2.5 cm depth range. Overpenetration of the REMOTS® camera prism precluded 

 determining the RPD boundary layer at Station F400N. 



The areal distribution of apparent RPD depths in the 1992 survey showed a 

 correlation between RPD depth and the proximity of the station to the designated disposal 

 point (Figure 3-16). Six of the seven WLIS "F" stations with apparent RPD depths 

 > 3.0 cm fell within 100 m of the WLIS "F" mound. In addition, the majority of stations 

 with apparent RPD depths of > 2.0 cm encompassed the WLIS "F" mound. These results 

 indicated that the deepest apparent RPD depths were associated with the recently deposited 

 dredged material. Apparent RPD depths for the 1991 survey were moderately developed 

 within 100 m of the WLIS "E" mound (the active disposal point for the 1990-1991 disposal 

 season); however, no clear relationship between RPD depth and proximity to the mound was 

 apparent. 



Many of the replicate photographs from stations F200W, F300W, and F400W showed 

 relatively dark subsurface sediments and associated shallow RPD layers (Figure 3-17). 

 These sediments may contain significant inventories of organic labile material which, when 

 processed by microbial activity, can result in decreased dissolved oxygen concentrations 

 within sediment pore waters. The sediments at these stations were considered to be a 

 mixture of past and recently deposited materials. 



Sixty percent of the apparent RPD depths for the 1992 pooled reference areas were in 

 the 1.0-2.5 cm depth range (Figure 3-18). The mean apparent RPD depths at reference areas 

 2000W, EAST, and SOUTH were 1.88, 1.59, and 1.93 cm, respectively. This unimodal 

 distribution of the 1992 RPD data contrasted with the bimodal 1991 RPD distribution and 

 reflected the relative consistency in the apparent RPD depths observed at the 1992 reference 

 areas. The 1991 bimodality was driven largely by the deep RPD depths noted at reference 

 area 2000W (mean 3.77 cm) relative to RPD depths calculated for WLIS-REF and 2000S 

 (mean 1.81 and 1.53 cm, respectively). 



Well-defined apparent RPD boundary layers characterized sediment-profile 

 photographs at the 2000W reference area in 1991, whereas RPD boundary layers were not as 

 distinct in the 1992 data. Overall, the moderate-to-high reflectance of 1992 sediment-profile 



Monitoring Cruise at the Western Long Island Sound Disposal Site, July 1992 



